1. Field of the Invention
The present invention relates to an improvement in processing efficiency and processing accuracy in a laser processing apparatus, especially a laser cutting apparatus.
2. Description of the Prior Art
In laser processing, especially laser cutting, a beam having a good focusing characteristic is required as a condition for performing high-accuracy processing. As a beam satisfying this condition, a beam of the TEM00 mode, whose focusing characteristic is theoretically the highest, has hitherto been used. On the other hand, however, the higher the order of the mode, the higher the output power that can be obtained, so that beam modes whose orders are higher than the zeroth order, e.g., TEM01*, have been used. The configurations of these beam modes are shown in, for instance, "Laser Processing Technology," page 19, published by The Nikkan Kogyo Shimbun, Ltd., and are reproduced herein in FIG. 20.
FIG. 19 is a schematic diagram illustrating an example of a conventional processing apparatus. In the drawing, reference numeral 1 denotes a laser oscillator; 2, a laser beam; 3, a processing head, such as a cutting head; 4, a condenser lens; 5, an X-Y table; 6, a controller; 7, a reflecting mirror; 8, a partial-reflection mirror (a beam-fetching window); 9, an output detection sensor; 10, a mode changeover unit; and 11, a workpiece.
Next, the operation of the apparatus will be described. The laser beam 2 emitted from the oscillator 1 is directed toward a location above the workpiece 11 by means of the reflecting mirror 7, and is focused on the workpiece 11 by means of the condenser lens 4. At this time, a processing assist gas is simultaneously injected through an opening at a lower end of the processing head 3 together with the laser beam 2. Here, the positional relationship between the focal point of the laser beam and the workpiece 11 (i.e., focal position), the assist gas pressure, and the nozzle height (the distance between a processing-head lower end and the workpiece) become important parameters at the time of processing. In addition, stored in a storage unit of the controller 6 are a plurality of processing conditions, such as the focal position, the processing gas pressure, the nozzle height, the laser output, the form of laser oscillation output (continuous wave or pulse wave), the pulse frequency, and the pulse duty cycle. Optimum conditions are selected in accordance with the material of the workpiece 11, plate thickness, processing (cutting) speed, and a processing (cutting) profile. The oscillator 1, the processing head 3, the X-Y table 5, and the like are controlled on the basis of these conditions. In addition, the beam mode of the laser beam 2 is changed by the mode changeover unit 10.
In the laser processing apparatus arranged in the above-described manner, if the output is increased in processing in the TEM00 mode, which gives a good focusing characteristic, defective processing and unstable processing are observed. To cite an example, in a case where piercing (causing the beam to pierce the workpiece) is effected with a low output at the start of processing, and the output is increased upon completion of piercing to effect the cutting of the workpiece, satisfactory processing can be effected during piercing and an early period of cutting, but defective processing can gradually occur as the cutting operation proceeds.
It has been confirmed that this occurrence is caused by the fact that the transmitting optical elements (the partial-reflection mirror 8 and the condenser lens 4) absorb part of the laser beam and undergo a temperature rise, so that distortion occurs in their configurations and in the distribution of refractive index, thereby resulting in the deterioration of the beam-focusing characteristic and in a change of the position of the focal point. This phenomenon is referred to as thermal strain, and occurs when such absorptance is present in the transmitting optical elements. In addition, if the absorptance of the transmitting optical elements becomes large due to the attachment of impurities to the surfaces of the transmitting optical elements, the amount of heat generated in the transmitting optical elements becomes even greater, and the deterioration of the beam-focusing characteristic due to thermal strain becomes more pronounced.
Accordingly, in continuous processing (e.g., cutting), the temperature rise of the transmitting optical elements is low and the effect of thermal strain is small during the early period of processing. However, as the processing proceeds, the temperature of the transmitting optical elements rises, and the deterioration of the beam-focusing characteristic and the movement of the position of the focal point become large, thereby inducing defective processing (cutting).
It has been found as a result of experiments that the amount of the above-described deterioration of the focusing characteristic and the amount of movement of the focal position are most pronounced in the TEM00 mode. Hence, it became evident that defective processing frequently occurred because the TEM00 mode was conventionally used for processing (e.g., cutting) for conditions where the TEM00 mode was possible, for the reason that the TEM00 mode theoretically gives the highest focusing characteristic.
For such conventional laser processing apparatuses, only rough criteria have been available for choosing the beam modes. For instance, a lower-order mode, e.g., a single mode which gives a good focusing characteristic, has been used insofar as an output can be secured, while a higher-order mode, which is readily capable of increasing the output, has been used for processing operations that require high output and for other similar purposes. Hence, there has been a drawback in that the beam modes most suitable for processing (e.g., cutting) have not necessarily been used.